Antennas that are used in mobile communication, satellite broadcasting, and the like require a beam scanning function that is capable of changing beam direction. Proposed examples of antennas having such a function include scanning antennas based on the high dielectric anisotropy (birefringence) of liquid crystals (including nematic liquid crystals and polymer-dispersed liquid crystals) (see, for example, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2013-539949, Japanese Unexamined Patent Application Publication (Translation of PCT Application.) No. 2016-512408, Japanese Unexamined Patent. Application Publication (Translation of PCT Application) No. 2009-538565, and International Publication No. 2015/126550).
This type of scanning antenna includes a configuration (i.e. a liquid crystal cell for use in a scanning antenna) in which a liquid crystal layer is sandwiched between two substrates provided with electrodes. On a surface of each electrode of the liquid crystal cell, a protective film constituted by a nitride film or the like or an alignment film constituted by a polyimide film or the like is formed.
A scanning antenna including such a liquid crystal cell controls directivity between the presence of the application of a voltage between electrodes and the absence of the application of a voltage. This control involves the use of the capacitance change properties (variability) of liquid crystals.
The scanning antenna has an antenna unit (antenna element) structured such that a first alignment film, liquid crystals, and a second alignment film are stacked in the order named. Assume here that Cpia is the capacitance of the first alignment film, Clc is the capacitance of the liquid crystals, and Cpib is the capacitance of the second alignment film. Then, simply put, the resultant capacitance C can be expressed as “1/C=1/Cpia+1/Clc+1/Cpib”.
Ideally, it is desirable that the antenna gain of the scanning antenna be the ON/OFF ratio in capacitance of Clc_on in the presence of the application of a voltage (ON) to Clc_off in the absence of the application of a voltage (OFF). However, since the protective films or the alignment films, which have insulation properties, are provided on the electrodes, a change Con/Coff in resultant capacitance as an antenna unit cannot avoid becoming smaller than the value of Clc_on/Clc_off. This has presented a problem.
It is conceivable that high-dielectric alignment films may be used so that “1/Cpia” and “1/Cpib” approximate zero. However, in order for the liquid crystals to achieve the innate antenna gain, the relative dielectric constant of all of the films must take on values of several tens to several hundreds. This is not realistic.